It is an object of the present invention to provide peptides which comprise a monosulfide bridge. This thioether bond is also designated as a lanthionine bridge and corresponds with the cystine bridge with the exception that the disulfide bridge is replaced by a monosulfide linkage. Two amino acid residues having the general formula 
are designated to be joined into a lanthionine bridge wherein the linkage of the two amino acids has the meaning xe2x80x94RCHxe2x80x94Sxe2x80x94HCRxe2x80x2xe2x80x94, wherein R and Rxe2x80x2 respectively represent xe2x80x94H, a lower (C1-C10) alkyl or aralkyl group. In a preferred embodiment R and Rxe2x80x2 are H. The amino acid termini of the lanthionine structure are designated as AlaL if R and Rxe2x80x2 are H and ThrL when R or Rxe2x80x2 are CH3. Other xcex2-substituted lanthionine components are designated as substituted AlaL derivatives, e.g. xcex2ethylAlaL.
Thioether bonds of the lanthionine type are known from some fungal toxins and antibiotics, for example from the lantibiotics, as nisin, epidermin, dunamycin or mersacidin. Naturally occurring compounds having the monosulfide bridge always have more than two monosulfide bridges in the molecule.
M. F. Bean et al. have reported in their article xe2x80x9cIdentification of a Thioether By-product in the Synthesis of a Cyclic Disulfide Peptide by Tandem Mass Spectrometryxe2x80x9d as published in the Proceedings of the 11th American Peptide Symposium, ESCOM, (Leiden 1990, p. 443) on a somatostatin analog wherein the internal disulfide bond has been converted to a thioether link. The somatostatin analog with the putative amino acid sequence Phe-Ala-Phe-Trp-Lys-Thr-Ala-Thr(ol), wherein the two AlaL residues are linked via the thioether bridge, has been described as the by-product which was obtained by the Boc-TFA-preparation of sandostatin analogs. The originally occurring somatostatin derivative has a disulfide bridge.